1. Field of the Invention
This invention relates to a method of and an apparatus for replacing control rod drives of nuclear reactors, and more particularly to a method of and an apparatus for replacing a control rod which enables the achievement of a compact design of a control rod drive replacing apparatus.
2. Description of the Prior Art
In order to drive control rods inserted in a pressure vessel of a nuclear reactor, the nuclear reactor is provided with a control rod driving mechanism including control rod drives each coupled to one each of the control rods in the pressure vessel. The control rod drive (hereinafter referred to as a CRD) is regularly inspected and maintained so that the operation of the control rods can be performed smoothly under trouble-free condition while the nuclear reactor is in operation.
In a boiling-water reactor, the CRDs are each mounted in a CRD housing attached to the bottom of the pressure vessel. Thus, difficulties are encountered in detaching each CRD from the pressure vessel and attaching the same again to the pressure vessel, for carrying out inspection and maintenance of each CRD. More specifically, a space in a pedestal below the pressure vessel in which the CRDs are mounted is one of the regions in which the level of radioactivity is very high, so that the aforesaid CRD attaching and detaching are carried out by the operator wearing protective clothing. This makes the attaching and detaching operation time-consuming and puts a strain on the operator who runs the risk of exposure to radiation.
Proposals have been made to mechanize the operation of attaching and detaching the CRD so as to automatically carry out a CRD replacing operation. One of such proposals is disclosed in U.S. Pat. Ser. No. 778,168, now U.S. Pat. No. 4,158,600, and Japanese Patent Application, Laid-open No. 140793/77 which describe a CRD replacing apparatus comprising a CRD attaching and detaching device and a CRD gripping device which are movable along a column which is pivotally mounted on a carriage arranged on a revolving carriage. The arrangement in which the CRD attaching and detaching device and the CRD gripping device are mounted on the column makes it necessary to move the CRD attaching and detaching device to the lowermost end of the column when a CRD is moved between the CRD attaching and detaching device and the CRD gripping device. As a result, it becomes necessary to increase the length of the column and hence to increase the distance between the bottom of the space within the pedestal in which the CRD replacing apparatus is installed and the lower end of each CRD attached to the respective CRD housing. This results in an increase in the height of the pedestal which supports the pressure vessel of the nuclear reactor, thereby deteriorating the ability of the nuclear reactor to withstand the shock of earthquakes. Also, the use of a column of a large length raises the problem of how to move the column to a horizontal position without any trouble. The solution of this problem entails an increase in the diameter of the space within the pedestal. Thus, the CRD replacing apparatus of the prior art becomes large in size, and the space within the pedestal for installing the apparatus therein should be increased. At the same time, it is not easy to move the CRD replacing apparatus of a large size from the space within the pedestal to outside, so as to effect inspection and maintenance.